Mock objects are objects which behave like another business objects, but they don’t do more than you need to test a unit. For example they response with a static data.
Mock frameworks gives you the ability to define dynamically mock objects in the test method rather than define them in separate classes. The mock object will behave like an object you need to test the unit you want.
For iOS there is a great mocking framework: OCMock (http://ocmock.org)